In conjunction with the development of digitalization in the information society, a demand has arisen for a writable medium that allows high density recording/reproduction.
In light of this a writable optical information recording mediums having various medium structures for improving recording capacities; and methods for realizing the high density recording/reproduction have been proposed. See the following examples (I) through (IV) of such methods:
(I) A method of shortening the wavelength of laser light for use in the recording/reproduction.
(II) A method of increasing NA (Numerical Aperture) of an objective lens for collecting light on an optical information recording medium.
(III) A method of providing a plurality of recording layers in an optical information recording medium.
(IV) A method of practically reducing a light spot diameter of laser light by forming, in an optical information recording medium, a mask layer (also referred to as “light shutter layer”) for shielding a part of the light.
Now, consider the proposed conventional method (IV) of the methods of arranging the medium. The method (IV) is such a method that: the part of the light is shielded so that the size of an aperture is practically reduced, with the result that resolution is improved. However, this causes decrease of intensity of a signal generated from a fine recording pit, with the result that influence of noise becomes great.
In light of this, the following methods (V) and (VI) are proposed in view of the method (IV):
(V) A method of providing a super resolution film using a nonlinear optical property for the sake of attaining a super resolution reproducing effect.
(VI) A method of providing a mask layer for improving the resolution by using a near field light, which is generated by causing localized plasmon diffusion by means of light irradiation and which has a wavelength shorter than that of the propagated light.
Each of the methods is not a mere method of shielding the part of the light so that the size of the aperture is practically reduced as described above, but is a method of using the light. Specifically, in the methods, the property of the light and the property of the film are changed so that a new effect is obtained. For this reason, the method (IV) can be labeled as an “aperture type method”, whereas each of the methods (V) and (VI) can be labeled as a “non-aperture type method”.
Conventionally, such an aperture type method as the method (IV) has been proposed. However, the non-aperture type methods such as the methods (V) and (VI) have been actively proposed recently for the purpose of attaining a better effect. Examples of the methods (IV), (V), and (VI) are respectively described in Patent document 1, Patent document 2, and Patent document 3.
Disclosed in Patent document 1 (Japanese Unexamined Patent Publication Tokukaihei 05-89511/1993 (published on Apr. 9, 1993)) is a super resolution reproducing method. The super resolution reproducing method employs a transparent substrate on which pits and a phase change material layer are formed. The phase change material layer can be crystallized after being melted. When light (readout light) for use in readout is irradiated to the phase change material layer, the phase change material layer is partially melted in the spot of the readout light, with the result that the phase change material layer is brought into a liquid phase. Accordingly, reflectance for the readout light is changed. This allows for super resolution reproduction. After the readout is finished, the phase change material layer is brought back to the crystal phase.
On the other hand, Patent document 2 (WO2002/058060 (published on Jul. 25, 2002)) discloses an optical disk apparatus that uses an optical disk having a side (side to which laser light is irradiated from the objective lens) in which a super resolution film is formed under a protective layer formed under a recording layer storing information, and that uses reflectance change in the recording material of the optical disk so as to carry out information recording/reproduction. The super resolution film is made of a microcrystalline compound conductor, and has an absorption edge close to a wavelength of reproduction light emitted from a reproduction optical system of the optical disk apparatus. Further, by causing crystals in the super resolution film to be aligned in a specific manner, the super resolution film is brought to have the nonlinear optical property with respect to the reproduction light. This allows for the super resolution effect. The super resolution effect is also obtained when using a substrate including (i) rises and recesses each corresponding to recorded information, and (ii) a super resolution film similar to the above super resolution film. This is because the super resolution film is brought to have the nonlinear optical property with respect to the reproduction light, also in this case.
Further, Patent document 3 (Japanese Unexamined Patent Publication Tokukai 2004-14093/2004 (published on Jan. 15, 2004) discloses a read-only optical disk (playback-only optical disk; reproduction-only optical disk) using a zinc oxide nano thin film allowing for the near field effect.
However, each of the techniques disclosed in Patent documents 1 through 3 suffers from the following problems.
With respect to Patent document 1, a material such as the phase change material of which the mask layer (light shutter layer) is made exhibits the mask effect when being melted by increasing the temperature of the material to a certain temperature or greater. The material thus melted is highly flowable, so that the composition and the shape of the material are likely to be changed from those in its initial state. Therefore, the mask layer provided in the optical information recording medium exhibits the mask effect when being melted by increasing the temperature of the mask layer to the certain temperature or greater. However, as the optical information recording medium is recorded and/or reproduced over and over, the composition and the shape of the mask layer are so changed that the mask effect is gradually deteriorated. Specifically, substantially no mask effect is exhibited after recording and/or reproducing the optical information recording medium several thousand times. For this reason, the optical information recording medium described in Patent document 1 is insufficient in terms of durability, stability, and reliability.
Further, conventionally proposed as a material having a property similar to that of the phase change material described in Patent document 1 is a thermochromism material. The thermochromism material is a material whose transmittivity is changed in response to a chemical structure change caused by absorption of heat. Specific examples of such a thermochromism material whose transmittivity is changed in response to the temperature change include: (i) an organic thermochromism material obtained by adding alkali to lactone, fluorane, or the like; (ii) an organic thermochromism material obtained by adding an organic acid to leuco pigment or the like. For example, Japanese Unexamined Patent Publication Tokukaihei 05-12715/1993 exemplifies, as the thermochromism material, a material only consisting of organic materials.
However, the chemical structure of each of such organic materials is also changed by absorption of heat, so that the organic materials are defective in terms of thermal stability. Therefore, as the recording and/or reproduction are carried out over and over, the mask layer is so deteriorated that the mask effect is gradually deteriorated. Specifically, substantially no mask effect is exhibited after carrying out recording and/or reproduction several thousand times. In other words, such durability for the repeated recording/reproduction is unpractical. Thus, as is the case with Patent document 1, the optical information recording medium having such a mask layer is insufficient in terms of the durability, stability, and reliability.
As such, each of the materials exemplified above is defective in terms of the thermal stability, and does not have practical durability for the repeated recording/reproduction.
In the meanwhile, the super resolution film described in Patent document 2 uses the linear optical property of each of the group II-VI compound semiconductors (group II compounds: Zn and Cd; group VI compounds: S, Se, and Te), for attainment of the super resolution reproduction. However, the following general problems arise in adopting each of the above materials as the nonlinear optical material: (1) highly pure and large crystals are required; (2) it is difficult to carry out crystal uniformity control upon film formation; (3) acceptability is low in a crystal orientation angle or the like; (4) strong light intensity is required for the attainment of the nonlinear optical property; (5) optical damage resistance is low; and the like.
In Patent document 2, for attainment of a great nonlinear optical property, the film formation needs to be carried out in such a manner that the nonlinear optical material is dispersed in a glass matrix which serves as a grain boundary layer and which contains SiO2 and the like. Further, grain diameters need to be controlled for attainment of a better nonlinear optical property.
However, for the formation of such a super resolution film made of the materials, a mixture target needs to be sputtered or several mixture targets need to be sputtered simultaneously.
Such a film forming method actually makes it difficult to solve the aforementioned problems. That is, the nonlinear optical material is thus mixed with the film forming materials, so that it is difficult to obtain highly pure and large crystals of the nonlinear optical material, or to secure uniformity of the crystals thereof. Moreover, the nonlinear optical material is deposited on the priming layer in which the materials of various types are mixed, so that the nonlinear optical material is influenced by the priming layer. This makes it difficult to obtain (i) stable growth of the crystals, and (ii) preferred crystal orientation. Moreover, the film forming method makes it difficult to carry out the grain diameter control.
Further, required for the attainment of the nonlinear optical property in the optical recording/reproduction of the optical information recording medium described in Patent document 2 is light whose light intensity is stronger than that of light currently used for the optical recording/reproduction. However, this optical information recording medium is weak in terms of the optical damage resistance, and therefore suffers from problems of reproduction stability and durability.
As described above, it is difficult to control the property of each material for use in the optical information recording medium described in Patent document 2. Moreover, the materials therefor are limited, so that freedom in medium designing is low. This is not practical.
In the meanwhile, the optical recording/reproduction of the optical disk disclosed in Patent document 3 uses the near field light whose light intensity is stronger than that of the light used in the normal optical recording/reproduction. In cases where irradiation of such light is carried out over and over for the recording/reproduction, the entire medium including the substrate and the thin film are likely to be deteriorated. Therefore, the optical disk suffers from problems in reproduction stability and durability. Further, the zinc oxide nano thin film needs to be sandwiched between light transmitting dielectric thin films, and the film thickness of each of the light transmitting dielectric thin films needs to be controlled very carefully at a precision of several-nm order. This makes it difficult to carry out the property control in the optical disk described in Patent document 3, and to carry out the medium designing thereof, and to achieve implementation thereof.